This invention relates generally to a white light illumination system, and specifically to a ceramic phosphor blend for converting UV radiation emitted by a light emitting diode (xe2x80x9cLEDxe2x80x9d) to white light.
White light emitting LEDs are used as a backlight in liquid crystal displays and as a replacement for small conventional lamps and fluorescent lamps. As discussed in chapter 10.4 of xe2x80x9cThe Blue Laser Diodexe2x80x9d by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs are fabricated by forming a ceramic phosphor layer on the output surface of a blue light emitting semiconductor LED. Conventionally, the blue LED is an InGaN single quantum well LED and the phosphor is a cerium doped yttrium aluminum garnet (xe2x80x9cYAG:Cexe2x80x9d), Y3Al5O12:Ce3+. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
However, the blue LEDxe2x80x94YAG:Ce phosphor white light illumination system suffers from the following disadvantages. The LED color output (e.g., spectral power distribution and peak emission wavelength) varies with the band gap width of the LED active layer and with the power applied to the LED. During production, a certain percentage of LEDs are manufactured with active layers whose actual band gap width is larger or smaller than the desired width. Thus, the color output of such LEDs deviates from the desired parameters. Furthermore, even if the band gap of a particular LED has the desired width, during LED operation the power applied to the LED frequently deviates from the desired value. This also causes the LED color output to deviate from the desired parameters. Since the light emitted by the system contains a blue component from the LED, if the color output of the LED deviates from the desired parameters, then the light output by the system deviates form the desired parameters as well. A significant deviation from the desired parameters may cause the color output of the system to appear non-white (i.e., bluish or yellowish).
Furthermore, the color output of the blue LEDxe2x80x94YAG:Ce phosphor system varies greatly due to frequent, unavoidable, routine deviations from desired parameters (i.e., manufacturing systematic variations) during the production of the LED lamp because the color output of the blue LEDxe2x80x94YAG:Ce phosphor system is very sensitive to the thickness of the phosphor. If the phosphor is too thin, then more than a desired amount of the blue light emitted by the LED will penetrate through the phosphor, and the combined LEDxe2x80x94phosphor system light output will appear bluish, because it is dominated by the output of the blue LED. In contrast, if the phosphor is too thick, then less than a desired amount of the blue LED light will penetrate through the thick YAG:Ce phosphor layer. The combined LEDxe2x80x94phosphor system will then appear yellowish, because it is dominated by the yellow output of the YAG:Ce phosphor.
Therefore, the thickness of the phosphor is a critical variable affecting the color output of the prior art system. Unfortunately, it is difficult to control the precise thickness of the phosphor during large scale production of the blue LEDxe2x80x94YAG:Ce phosphor system. Variations in phosphor thickness often result in the system output being unsuitable for white light illumination applications, causing the color output of the system to appear non-white (i.e., bluish or yellowish), which leads to an unacceptably low blue LEDxe2x80x94YAG:Ce phosphor system manufacturing yield.
The blue LEDxe2x80x94YAG:Ce phosphor system also suffers from the halo effect due to the separation of blue and yellow light. The LED emits blue light in a directional fashion. However, the phosphor emits yellow light isotropically (i.e., in all directions). Therefore, when the light output by the system is viewed straight on (i.e., directly at the LED emission), the light appears bluish-white. In contrast, when the light output is viewed at an angle, the light appears yellowish due to the predominance of the yellow phosphor emission. When the light output by such a system is directed onto a flat surface, it appears as a yellowish halo surrounding a bluish area. The present invention is directed to overcoming or at least reducing the problems set forth above.
In accordance with one aspect of the present invention, there is provided a white light illumination system comprising a radiation source, a first luminescent material having a peak emission wavelength of about 570 to about 620 nm, and a second luminescent material having a peak emission wavelength of about 480 to about 500 nm, which is different from the first luminescent material.
In accordance with another aspect of the present invention, there is provided a white light illumination system comprising a light emitting diode having a peak emission wavelength between 370 and 405 nm, a first APO:Eu2+,Mn2+ phosphor, where A comprises at least one of Sr, Ca, Ba or Mg, and a second phosphor selected from at least one of:
a) A4D14O25:Eu2+, where A comprises at least one of Sr, Ca, Ba or Mg, and D comprises at least one of Al or Ga;
b) (2AO*0.84P2O5*0.16B2O3): Eu2+, where A comprises at least one of Sr, Ca, Ba or Mg;
c) AD8O13:Eu2+, where A comprises at least one of Sr, Ca, Ba or Mg, and D comprises at least one of Al or Ga;
d) A10(PO4)6Cl2:Eu2+, where A comprises at least one of Sr, Ca, Ba or Mg; or
e) A2Si3O8*2ACl2:Eu2+, where A comprises at least one of Sr, Ca, Ba or Mg.
In accordance with another aspect of the present invention, there is provided a method of making a white light illumination system, comprising blending a first phosphor powder having a peak emission wavelength of about 570 to about 620 nm and a second phosphor powder having a peak emission wavelength of about 480 to about 500 nm to form a phosphor powder mixture, and placing the phosphor powder mixture into the white light illumination system adjacent a radiation source.